I worked for home depot lumber department. Most of the lumber doesn't come from China like that other guy said. The only wood products from China in the lumber department are Luan underlayment and wood finials. These products are built specifically for home depot in assemblies in China, not by a 3rd party vendor. Pressure treated lumber comes from different vendors depending on area, but all vendors (I.e. prowood, yellawood, etc.) compete to buy from mills in the Pacific northwest and Canada. Home depot, Lowe's, ace hardware, etc. all get their wood from the same mills, the only difference is the companies that distribute that lumber. As far as 2x whitewood goes, it generally comes from Canada (canfor). Sheathing and plywood is typically locally sourced, coming from nearby lumber mills. Hardwood plywood and certain primed boards will be sourced from South America. Oh and 1x8, etc. "Common boards" generally come from Sweden.

So, basically the wood comes from wherever they can get the best price.